Runaway Gun
Nowadays I am happy at up to 14 K without oxygen but in the past did have a nasty situation in the very high teens over the Alps where I could not descend ran out of oxygen partially due to a leak and very strong unforecast headwinds. I had No choice but to stay there till the terrain lowered ( am still here although the girlfriend argues that point with me sometimes
)
Tolerance is a very individual thing
Take care
Pace